It is generally known from the prior art to produce thin slabs by the continuous casting method. This involves producing a molten metal, which is transferred into a tundish by means of a steel casting ladle. From the tundish, the molten metal flows by way of a casting tube into a mold, which is cooled and moved in an oscillating manner. In the mold there forms from the molten metal a strand with a solidified shell and a mostly not yet solidified cross section within the solidified shell. When it leaves the mold, the strand is taken up by a transporting system with a multiplicity of strand guiding rollers, between which the strand is passed through the so-called casting bow and is cooled down until it has solidified through completely. It is also known to slow down the flow rate of the molten metal inside the already partially solidified strand within the mold by means of an electromagnetic brake (EMBR). The aim here is to reduce the flow rate of the molten steel at the bath level and make the bath level profile more uniform, in order to improve the lubrication between the strand and the mold and reduce strand surface defects that may be caused by casting slag becoming entrapped.
For producing the thin slabs with thicknesses of between 40 and 120 millimeters, the mold typically has in the upper part a cross section that is widened in the form of a funnel and in the lower part a cross section that is rectangular. On account of these small thicknesses, the solidifying-through times in the case of thin-slab continuous casting are relatively short and the proportion of liquid material inside the partially solidified strand is low. This inevitably results in a coarse, highly directional, columnar crystalline microstructure in the continuous casting of thin slabs. Such a microstructure may however have disadvantageous effects on the quality of the surface and the interior of the products produced from the thin slabs. For example, depending on the grade of steel and the casting conditions, longitudinal striations on the product surface, inhomogeneous mechanical properties, microstructural stringers, core segregations, reduced HIC resistance (Hydrogen Induced Cracking) and internal crack susceptibilities may occur on the products produced from the thin slab material.
It is known from conventional thick-slab continuous casting to avoid longitudinal striations in the case of dynamo steels by casting with very low overheating. In the case of thick-slab continuous casting, however, there is a comparatively long solidifying-through time, so that overheatings of the molten steel in the tundish below about 12 kelvins are sufficient to achieve adequate microstructural refinement. The microstructural refinement can be described as adequate if the extent of the globular core zone in the thickness direction is more than 30%. In order to achieve the same effect in the case of thin slabs, the shorter solidifying-through times mean that such a low overheating would have to be chosen that casting problems in the form of clogging of the immersion tubes in the mold would occur, which could result in strand surface defects or even strand ruptures.
It is also known from the specialist literature (for example “Improved quality and productivity in slab casting by electromagnetic braking and stirring”, C. Crister et al., 41st Steelmaking Seminar International, Resende, Brazil, May 23-26, 2010, pages 1-15) that electromagnetic stirrers are used in some thick-slab continuous casting installations for the refinement of the solidification structure. The stirrers are installed here either in the region of the mold or several meters below the bath level of the mold.
The document DE 698 24 749 T2 also discloses a device for casting metal which comprises a mold for forming a cast strand and means for feeding a primary flow of hot molten metal to the mold. The device concerned has a magnetic system which applies a static or periodic magnetic field to the flow of the metal in the unsolidified parts of the cast strand, in order to act on the molten metal in the mold during the casting. This is intended to brake and divide up the flow of the hot metal, in order to achieve a secondary flow pattern in the mold. It is additionally known from this document to provide a further device in the form of an electromagnetic stirrer, in order to act on the molten material in the mold or on the molten material downstream of the mold. However, in this document it is not disclosed in which region with respect to the mold the electromagnetic stirrer is to be arranged.
The use of an electromagnetic brake and/or an electromagnetic stirrer in the continuous casting of steel is also known for thick-slab formats from the documents DE 21 2009 000 056 U1 and DE 10 2009 056 000 A1.
Electromagnetic stirrers have not so far been used in the continuous casting of thin slabs. The particular difficulty in thin-slab continuous casting is that of achieving a significant microstructural refinement with the short solidifying-through times in comparison with thick-slab continuous casting and the small-volume proportion of liquid inside the strand. The present invention solves this problem.